Note: Descriptions are shown in the official language in which they were submitted.
CA 02774106 2012-03-13
TWO-STAGE METHOD FOR THE CORROSION PROTECTION TREATMENT
OF METAL SURFACES
[0002] The present invention relates to an at least two-stage method for
corrosion-protective treatment of metal surfaces, in which method, in a first
step (i) an organic coating made up of an aqueous phase (A) is applied onto
the metal surface, and in a subsequent step (ii) the organic coating applied
onto the metal surface is brought into contact with an acidic aqueous
composition (B) that comprises at least one or more water-soluble compounds
containing at least one atom selected from the elements Zr, Ti, Si, Hf, V,
and/or Ce, and one or more water-soluble compounds that release copper
ions. The present invention furthermore encompasses a metallic component
that is produced at least partly from steel, iron, zinc, and/or aluminum and
alloys thereof and has been treated using the method according to the present
invention, and the use thereof in automobile construction and the construction
sector, and for the manufacture of household appliances and electronics
housings.
[0003] In the automotive industry, the corrosion-protective application of
paint systems made up of aqueous binding agent dispersions during body
production is existing art. The automotive industry makes use principally of
dip
coating, in which the basic bodies, pretreated in corrosion-protective
fashion,
are introduced in a continuous process into a dip tank containing a dispersed
paint system, deposition of the paint occurring either by application of an
external voltage (electrodip coating) or in autodepositing fashion merely as a
result of contact with the metal surfaces (autophoretic dip coating). The body
then experiences a heat treatment so that film formation and crosslinking of
the
paint system deposited on the metal surface occurs, ensuring a high level of
corrosion protection and allowing subsequent application of further coatings.
[0004] Autophoretic baths thus serve for the organic coating of metallic
surfaces, usually iron surfaces, as a corrosion-protective primer coating on
metallic components, or as an adhesive intermediate layer in the manufacture
CA 02774106 2012-03-13
of metal-elastomer composites, for example for vibration-damping components
in the automotive industry. Autophoretic coating is therefore a dip coating
process that, in contrast to electrodip coating, takes place in electroless
fashion, i.e. without application of an external voltage source. The
autodeposition compositions are usually aqueous dispersions of organic resins
or polymers which, upon contact with the metallic surface, coagulate in a thin
liquid layer directly at the surface of the component as a result of pickling-
based removal of metal cations, and thereby cause layer growth.
[0005] The use of autophoresis baths for dip coating deposition has lately
become more important in automobile production and especially in parts-
related production of metallic preforms, for example organic initial coating
of
wheel rims. Especially in the case of dip coating by means of autophoretically
acting resp. so-called autodeposition compositions, however, a post-treatment
is necessary in order to "heal" defects in the organic coating prior to a heat
treatment that crosslinks the paint.
[0006] In order to improve the corrosion resistance of the organic coatings
applied onto the metal surface using autophoretic methods, the existing art
proposes an aqueous reaction rinse subsequent to the organic initial coating
with the dip coat.
[0007] One such reaction rinse corresponds, according to
DE 10 2007 059969, to a passivating post-treatment of the uncrosslinked
coating, and brings about an inorganic conversion of the bare metal surface at
so-called micro-defects, for example with the aid of phosphate-containing
solutions that can furthermore contain alkali and/or alkaline-earth cations
and
also transition-metal cations, as well as fluoro complexes thereof.
[0008] US 6,410,092 accordingly discloses a chromium-free reaction rinse
based on water-soluble alkaline-earth metal salts, by preference calcium
nitrate, while in WO 02/42008, water-soluble salts of metals of groups Ila and
Ilb, by preference zinc salts, are used; in addition, soluble phosphates and
so-
2
CA 02774106 2012-03-13
called accelerators (which have an oxidizing effect) are said to be contained
in
the reaction rinse.
[0009] Proceeding from this existing art, the object of the present invention
is to develop a method for the initial deposition of hardenable organic
binding
agent systems onto metal surfaces and from an aqueous phase, in such a way
that the corrosion resistance of the metal surface protected by the cured
organic binding agent system is further improved.
[0010] The object is achieved by means of a multi-stage method for
corrosion-protective treatment of metal surfaces, in which method, in a first
step (i) an organic coating made up of an aqueous phase (A) is applied onto
the metal surface, wherein the metal surface having the organic coating is, in
a
subsequent step (ii), brought into contact with an acidic aqueous composition
(B) that comprises at least
a) one or more water-soluble compounds containing at least one atom
selected from the elements Zr, Ti, Si, Hf, V, and/or Ce, and
b) one or more water-soluble compounds that release copper ions.
[0011] The metal surface that is equipped in a first step (i) with an organic
coating can represent a bare metal surface that, in a cleaning and/or pickling
step preceding the method according to the present invention, has organic
contaminants removed from it. A bare metal surface of this kind is notable for
the fact that it is largely free of organic contaminants, for example
corrosion
protection oils, and there exists on its surface no (or only an ultra-thin)
oxide
covering layer that is made up of metallic elements of the metallic substrate
and has a layer thickness of only a few nanometers.
[0012] Metal surfaces according to the present invention are, however, also
those surfaces that have experienced, before the method step (i) according to
the present invention, a conversion treatment during which an inorganic
covering layer was formed. Inorganic conversion layers of this kind can be
made up of both metallic elements of the metal substrate and extraneous
metals. Typical conversion coatings are produced upon contact between bare
3
CA 02774106 2012-03-13
metal surfaces and acidic aqueous solutions that contain water-soluble
compounds of the elements Zr, Ti, Si, Hf, V, Ce, Mo, Zn, Mn, Fe, and in
addition, optionally, anions that form poorly soluble salts, such as
phosphates,
and/or complexing ions such as fluoride ions. The conversion treatment
produces amorphous or crystalline inorganic covering layers on the metal
surface; metal surfaces are still in accordance with present invention, and
can
be used for the method according to the present invention, if the layer weight
per unit area of the inorganic covering layers is equal to no more than 3
g/m2.
[0013] An organic coating that is applied onto the metal surface in the first
method step (i) is in accordance with the present invention if it contains a
hardenable organic binding agent system. The method step (i) according to the
present invention encompasses only the application of this organic coating,
but
not curing thereof by means of additional technical actions in order to
crosslink
the binding agent system. Additional technical actions of this kind are, for
example, heat treatment (thermal curing) or actinic irradiation (radiation
hardening) of an organic coating, applied in step (i), that contains the
hardenable binding agent system. The method step (i) does, however,
optionally encompass a heat treatment of the metal surface treated with the
aqueous phase (A) in order to evaporate some of the water that remains in the
wet film on the treated metal surface, even though the heat treatment has
been performed below the curing temperature of the organic binding agent
system. The organic coating that was applied out of the aqueous phase (A)
therefore also contains a portion of water. The organic coating can
furthermore
contain leveling agents, surfactants, corrosion inhibitors, salts, pigments,
and
other active substances and adjuvants known to one skilled in the art of
coatings technology. The solids content of the organic coating is, however,
equal to at least 20 wt%. An "organic coating" is understood as that portion
of
a wet film of the aqueous phase (A) containing a hardenable organic binding
system, applied in step (i), which remains on the metal surface, after a
rinsing
step under running water immediately subsequent to step (i), as a permanently
adhering film containing the hardenable organic binding agent system.
4
CA 02774106 2012-03-13
[0014] Deposition of the organic coating in step (i) of the method according
to the present invention occurs from an aqueous phase (A). The type of
deposition is not linked to specific technical actions, however, and it can
occur
by electrodip coating of the metal surface or by electroless methods such as
autophoretic deposition and the mechanical application methods known in the
existing art (roller application methods, spray methods).
[0015] It is, however, in particular in the context of electroless deposition
of
the organic coating in method step (i) from an aqueous phase (A) that the
method according to the present invention exhibits the most significant
improvement in the corrosion resistance of the metal surfaces treated in the
method according to the present invention. Those methods according to the
present invention in which application of the organic coating in the first
step (i)
occurs in electroless fashion, in particular autophoretically, by bringing the
metallic surface into contact with an aqueous phase (A) containing the organic
binding agent, are accordingly preferred.
[0016] If what occurs in the first step (i) of the method according to the
present invention is autophoretic deposition of the organic coating onto the
metal surface, then the aqueous phase (A) preferably has a pH of less than 4
and preferably contains
a) at least one dispersed organic binding agent system that is thermally
hardenable, by preference at temperatures below 300 C, by preference
below 200 C,
b) iron(III) ions, and
c) fluoride ions in a quantitative proportion such that the molar ratio of
fluoride
ions to iron(III) ions from water-soluble compounds is equal to at least
2: 1.
[0017] For an autophoretic deposition of this kind, the aqueous phase (A) in
step (i) of the method according to the present invention preferably contains
at
least 1 wt% of the organic binding agent system.
CA 02774106 2012-03-13
[0018] "Thermally hardenable" organic binding systems are those binding
agent systems that possess curing temperatures above 20 C and below the
indicated temperatures of 300 C, by preference below 200 C.
[0019] The "curing temperature" is the highest temperature that, in a
dynamic differential calorimetric analysis (DSC) over a temperature range from
20 C to 400 C at a heating rate of 10 K/min of a solids mixture of the organic
binding agent systems used, denotes the maximum of an exothermic process.
Calorimetric analysis of the exothermic quantities of heat released from the
sample volume of the solids mixture and recorded by DSC is accomplished in
accordance with DIN 53 765 in consideration of DIN EN ISO 11357-1. A solids
mixture of the organic binding agent system used is accessible by vacuum
freeze-drying of an aqueous dispersion of the binding agent system.
Alternatively, the aqueous dispersion of the binding agent system can be dried
at room temperature in the sample crucible for DSC measurement, and the
sample weight of solids mixture in the sample crucible can be ascertained by
differential weighing. The aqueous phase (A) is particularly suitable as an
aqueous dispersion.
[0020] Thermally crosslinkable resp. hardenable organic binding agent
systems in accordance with component a) of aqueous phase (A), which are
deposited in step (i) of a method preferred according to the present invention
in
electroless fashion by autophoretic deposition onto the metal surface, are
made up of organic oligomeric or polymeric compounds having at least two
functional groups, and are consequently capable of reacting with one another
in condensation or addition reactions with the formation of covalent bonds,
and
thereby building up a network of covalently linked oligomeric or polymeric
compounds. Thermally crosslinkable resp. hardenable binding agent systems
can be made up either of a self-crosslin king oligomeric or polymeric compound
having two different or identical functional groups capable of reacting with
one
another, or of at least two different oligomeric or polymeric compounds that
crosslink with one another as a result of their functionalization.
6
CA 02774106 2012-03-13
[0021] The organic binding agent system dispersed in water in accordance
with component a), which is applied in step (i) of a method according to the
present invention in electroless fashion onto the metal surface, contains at
least one thermally self-crosslinking organic polymer and/or a mixture of at
least one crosslinkable organic polymer resp. a resin and an organic hardener
that can react with the crosslinkable functionalities of the organic polymer
resp.
the resin in an addition or condensation reaction. The organic hardener can
likewise be an organic polymer resp. a resin.
[0022] For sufficient filming of the hardenable binding agent system on the
metal surface, it is further preferred that the organic binding agent system
dispersed in the aqueous phase (A) in step (i) of the method according to the
present invention have a film formation temperature of no more than 80 C,
particularly preferably no more than 40 C. If the film formation temperature
of
the binding agent is above the preferred 80 C, this can result in an
inhomogeneous organic coating of the metal surface during the reaction rinse
with an acidic aqueous composition (B) in step (ii) of the method according to
the present invention, which cannot be remedied even in the curing process
that usually follows the method according to the present invention. This kind
of
inhomogeneous coating of the metal surface with the organic binding agent
system has a disadvantageous effect on the corrosion resistance and visual
impression of the coated metal surface.
[0023] Because it is advantageous that the organic binding agent system
deposited in step (i) onto the metal surface form a film already during the
reaction rinse in step (ii), those methods according to the present invention
in
which the acidic aqueous composition (B) is brought, in step (ii), into
contact
with the metal surface having the organic coating at a temperature of at least
30 C, particularly preferably at least 40 C, but by preference no more than
80 C, are preferred.
[0024] The dispersed organic binding agent system used in step (i) of the
method preferred according to the present invention for electroless deposition
is by preference made up of at least one copolymerizate and/or polymer
7
CA 02774106 2012-03-13
mixture of acrylates with at least one oligomeric and/or polymeric compound
selected from epoxy resins, phenol resins, and/or polyurethane resins.
[0025] Water-dispersible epoxy resins bring about, as a crosslinked coating
on a metal surface, a particularly good barrier effect with respect to
corrosive
media, and are therefore a preferred constituent of the dispersed binding
agent
system in a method preferred according to the present invention in which, in
step (i), the organic coating is applied in electroless fashion, i.e. via an
autodeposition process. Optionally, crosslinking hardeners, preferably based
at
least in part on phenol resins, can be used in addition to the epoxy resin in
order to accelerate the curing process and increase the degree of
crosslinking.
Further hardeners that crosslink the epoxy resin are those based on
isocyanate resins, the isocyanate groups of which can also be present in
blocked fashion. Moderately reactive isocyanates are preferred as preferred
blocked isocyanate resins, for example aliphatic isocyanates and sterically
hindered isocyanates and/or isocyanates blocked in acid-stable fashion.
[0026] It is also possible to use, as an epoxy resin, incompletely crosslinked
oligomeric or polymeric compounds having free, for example terminally
bonded, epoxy groups, the preferred molecular weight of which is no less than
500 u and no greater than 5000 u. Examples of such epoxy resins are those
based on bisphenol A and bisphenol F, as well as epoxy-phenol novolacs.
[0027] For reasons of economy and commercial availability, epoxy resins
based on bisphenol A, which correspond to the following general structural
formula (III):
0
(A)n O (III)
0
are preferably used in the context of the present invention.
8
CA 02774106 2012-03-13
[0028] The structural module A corresponds here to the following general
formula (IV):
OH
O O (IV)
where n is a whole number from 1 to 50.
[0029] Preferred epoxies have an epoxy equivalent weight (EEW) of no less
than 100 g/eq, but no more than 5000 g/eq. The EEW indicates here the
average molecular weight per mol of epoxy functionality in the epoxy resin, in
grams per molar equivalent (g/eq). Particularly preferred ranges for the epoxy
equivalent weight exist for specific epoxy resins:
Brominated epoxy resins 300 to 100 g/eq, in particular 350 to 600
Polyalkylene glycol epoxy resins 100 to 700 g/eq, in particular 250 to 400
Liquid epoxy resins 150 to 250 g/eq
Solid/pasty epoxy resins 400 to 5000 g/eq, in particular 600 to 100
[0030] As phenol resins, incompletely crosslinked oligomeric or polymeric
polycondensation products of formaldehydes with phenols can be present in
dispersed fashion in the aqueous phase (A) in step (i) of the preferred method
according to the present invention for electroless deposition of the organic
coating, said products preferably comprising at least partly etherified
hydroxyl
groups and their preferred average molecular weight being no less than 500 u
and no greater than 10,000 u. The hydroxyl groups are present, in this
context,
by preference in methoxylated, ethoxylated, propoxylated, butoxylated, or
ethenyloxylated fashion. Both resols and novolacs can be used as phenol resin
types.
[0031] Further optional constituents of the aqueous phase (A) that, upon
contact with metal surfaces, bring about an autophoretic deposition of an
organic coating as defined by this invention are leveling agents, such as
glycol
ethers and alcohol esters, for better film formation of the deposited organic
9
CA 02774106 2012-03-13
coating on the metallic surface, micronized inorganic fillers such as
sulfates,
oxides, and phosphates having average particle sizes below 5 pm, by
preference below 1 pm, to increase the scratch resistance and corrosion
resistance of the organic coating in the cured state, as well as pigments for
coloring, for example Aquablack 255A of Solutions Inc.
[0032] With regard to composition (B) of the reaction rinse in step (ii) of
the
method according to the present invention, it has been possible to ascertain
that acidic aqueous compositions (B) containing
a) at least a total of 100 ppm, but no more than 2000 ppm, water-soluble
compounds containing at least one atom selected from the elements Zr, Ti,
Si, Hf, V, and/or Ce, calculated as a proportion of the respective element,
in particular no more than 800 ppm water-soluble compounds containing at
least one atom selected from the elements Zr, Ti, and/or Si, particularly
preferably Zr and/or Ti, calculated as a proportion of the respective
element, and
b) at least 1 ppm, but no more than 100 ppm, in particular no more than
50 ppm, water-soluble compounds that release copper ions, calculated as
a proportion of copper,
are preferred.
[0033] If the proportion of water-soluble compounds in accordance with
component a) is much below the preferred value, the "healing" of defects in
the
organic coating deposited from the aqueous phase then does not occur
sufficiently, and an additional positive effect due to the presence of the
compounds in accordance with component b) that release copper ions is
absent.
[0034] Conversely, it has been found that if the quantity of copper-ion-
releasing compounds in accordance with component b) falls considerably
below the preferred value, the reaction rinses obtained are ones that result
in
no improvement in the corrosion resistance of the metal surface equipped with
the cured organic binding agent system, as compared with reaction rinses
known in the existing art and made up exclusively of compounds in accordance
CA 02774106 2012-03-13
with component a). The addition of small quantities of copper-ion-releasing
compounds to a reaction rinse (B) containing a component a) does, however,
already bring about a considerable increase in the corrosion resistance of a
metal surface that has been treated according to method step (i). Quantities
of
copper-ion-releasing compounds above 50 ppm, based on copper, do not
contribute further to an increase in corrosion resistance and are therefore
uneconomical, while greater additions above 100 ppm once again cause a
slight degradation in corrosion resistance.
[0035] The reaction rinse to be performed in step (ii) of the method
according to the present invention, by bringing it into contact with the metal
surface having the organic coating, occurs by preference at a pH value for the
acidic aqueous composition (B) of no lower than 2 and no higher than 5. Lower
pH values can, depending on the organic binding system used, chemically
modify the organic coating and initiate decomposition reactions. In addition,
elevated acid corrosion of the metallic substrate, and the formation of
nascent
hydrogen, can permanently damage the interface between the metal and the
organic coating. Compositions having pH values above 5 are also less
preferred because the compositions (B) tend to form poorly soluble
precipitates
as a result of hydrolysis reactions of the water-soluble compounds in
accordance with components a).
[0036] For improved complexing of the metal cations that are dissolved out
of the metal substrate carrying the hardenable organic coating as a result of
the pickling process, in the method according to the present invention in step
(ii) fluoride ions can additionally be contained in the acidic aqueous
composition (B). Preferably, however, the proportion of fluoride ions in
composition (B) does not exceed values for which the measured free fluoride
proportion is higher than 400 ppm, although for an intensified pickling effect
on
the substrate and effective complexing of the metal cations, at least 1 ppm
free
fluoride should be present in composition (B). Hydrogen fluoride, alkali
fluorides, ammonium fluoride, and/or ammonium bifluoride serve, for example,
as a source of fluoride ions.
11
CA 02774106 2012-03-13
[0037] Preferred water-soluble compounds of component a) in step (ii) of
the method according to the present invention are compounds that dissociate
in aqueous solution into anions of fluoro complexes of the elements zirconium,
titanium, and/or silicon, particularly preferably fluoro complexes of the
elements zirconium and/or titanium. Preferred compounds of this kind are, for
example, H2ZrF6, K2ZrF6, Na ZrF6, and (NH4)2ZrF6, and the analogous titanium
resp. silicon compounds. Fluorine-containing compounds of this kind in
accordance with component a) are at the same time a source of free fluoride.
Fluorine-free compounds of the elements titanium and/or zirconium can also
be used according to the present invention as water-soluble compounds in
accordance with component A), for example (NH4)2Zr(OH)2(CO3)2 or TiO(SO4).
[0038] Preferred water-soluble compounds of component b) in step (ii) of
the method according to the present invention are all water-soluble copper
salts that contain no chloride ions. Copper sulfate, copper nitrate, and
copper
acetate are particularly preferred.
[0039] The acid compositions used in step (ii) of the method according to
the present invention can additionally contain so-called "depolarizers," which
as a result of their mild oxidizing effect suppress the formation of nascent
hydrogen at bare metal surfaces during the reaction rinse. The addition of
such
depolarizers, which are known in the technical field of phosphating of metal
surfaces, is therefore likewise preferred according to the present invention.
Typical representatives of depolarizers are chlorate ions, nitrite ions,
hydroxylamine, hydrogen peroxide in free or bound form, nitrate ions, m-
nitrobenzenesulfonate ions, m-nitrobenzoate ions, p-nitrophenol, N-
methylmorpholine-N-oxide, nitroguanidine.
[0040] For environmental reasons and in order to avoid inorganic heavy-
metal-containing sludges that must be laboriously processed and disposed of,
the use of water-soluble phosphates and chromates in the acidic aqueous
composition (B) of the reaction rinse in step (ii) is largely omitted. A
composition (B) in the reaction rinse, i.e. in step (ii) of the method
according to
the present invention, preferably contains no more than 1 ppm soluble
12
CA 02774106 2012-03-13
phosphates and chromates, calculated as the sum of P04 and Cr04,
particularly preferably no soluble phosphates and chromates. The present
invention is moreover notable for the fact that the presence of soluble
phosphates in step (ii) of the method can be dispensed with, but that
outstanding corrosion resistance in the metal substrates treated according to
the present invention nevertheless results.
[0041] The operation of bringing the aqueous phase (A) in step (i), and the
acidic aqueous composition in step (ii), into contact with the metal substrate
or
the metallic component occurs, in the method according to the present
invention, preferably in a dip or spray method, the dip method being
particularly
preferable because of the more homogeneous wetting of the surface.
[0042] To avoid the drag-over of constituents of the aqueous phase (A)
from step (i) into the acidic aqueous composition (B), those methods according
to the present invention in which a rinsing step occurs between the first step
(i)
and the subsequent step (ii), in order to remove components of the aqueous
phase (A) from the treated metal surface, are preferred. This action moreover
increases the effectiveness of the reaction rinse with the acidic aqueous
composition (B), since polymer particles that are not, or are insufficiently,
adhering to the metal surface are removed, so that the acidic aqueous
composition can act directly on the permanently adhering organic coating.
[0043] The contact times with the respective aqueous compositions are not
critical for the method according to the present invention, but in step (i)
should
preferably be selected so that the layer weight of the uncured, but
permanently
adhering, organic coating applied in step (i) of the method according to the
present invention is equal, immediately before the reaction rinse with the
acidic
aqueous composition (B) in step (ii), by preference to at least 10 g/m2,
particularly preferably at least 20 g/m2, but by preference no more than 80
g/m2. Experience indicates that lower layer weights result in inhomogeneous
coatings that impart a lower level of corrosion resistance to the metal
surface,
while higher layer weights do not substantially improve the corrosion
resistance
of the coated metal substrate. The layer weight of the uncured but permanently
13
CA 02774106 2012-03-13
adhering organic coating is determined after rinsing the metal substrate
coated
in step i) of the method according to the present invention under running
deionized water, the rinse being carried out until the rinse water flowing off
the
metal substrate does not appear turbid.
[0044] The contact times for the reaction rinse with the acidic aqueous
composition (B) to be performed in step (ii) of the method according to the
present invention are by preference 50 to 100% of the contact time with the
aqueous phase (A) in step (i).
[0045] The organic coating that is applied onto the metal surface in step (i)
and post-treated in step (ii) is by preference cured at elevated temperature,
with or without an interposed rinsing step in order to remove components of
the
acidic aqueous composition (B) from the treated metal surface, in order to
crosslink the polymeric coating as completely and permanently as possible and
thereby enhance corrosion resistance. The process of curing the organic
coating is carried out preferably at temperatures above the curing temperature
of the binding agent dispersed in the aqueous phase (A), and below 300' C.
[0046] The present invention also encompasses the metallic component
manufactured in the method according to the present invention, the component
by preference being produced at least partly from steel, iron, zinc, and/or
aluminum as well as alloys thereof.
[0047] A component of this kind according to the present invention is
utilized in automobile construction and the construction sector, and for the
manufacture of household appliances and electronics housings.
14
CA 02774106 2012-03-13
EXEMPLIFYING EMBODIMENTS
[0048] The effect of the reaction rinse performed in step (ii) of the method
according to the present invention in improving the corrosion resistance of
the
coated metal substrate is presented below by way of example for specific
organic binding agent systems that are applied onto steel surfaces using an
autophoretic process.
[0049] The CRS panels were first degreased for 7 minutes with a highly
alkaline cleaner (3 wt% ACL 1773, 0.3 wt% ACL 1773T, Henkel Co.), and
then washed with tap water and deionized water.
[0050] The panels were then immersed for 2 minutes into the respective
autodeposition bath for application of the organic coating (step i), then
rinsed
under running deionized water for one minute, and post-treated in step (ii)
for
one minute in a reaction rinse (ARR E2, Henkel KGaA) and again rinsed with
deionized water.
[0051] In a subsequent step, the panels coated in this fashion were filmed
and hardened in a recirculating oven. The layer thickness after curing, both
for
the method according to the present invention and in the comparison
experiments, was approx. 20 pm, and was determined using a PosiTector
(DeFelsco Corp.).
[0052] This was followed by quantification of the corrosion resistance of the
steel panels coated and treated in this fashion, based on infiltration in the
DIN
50021 NSS test. The results thereof are listed in Table 1.
[0053] The organic coatings applied in step (i) onto the steel surface in an
autophoretic process, from aqueous autodepositing dispersions of the
respective binding agent system, are all based on a polymer mixture of epoxy
resin (EEW: 500 to 575 g/eq; Mn: 1200 g/mol DER 664 UE, Dow Chemicals)
and polyacrylates, additionally containing a quantity of a hardener such that
the
weight ratio of epoxy resin is in each case 70:30. The organic solids content
of
CA 02774106 2012-03-13
the aqueous dispersions is approx. 4 wt%, and the proportion of epoxy resin in
the solids portion is approx. 45 wt%. In addition, 0.14 wt% iron(III)
fluoride,
0.05 wt% hydrogen fluoride, and 2.1 wt% hydrogen peroxide are contained in
the aqueous phase for autophoretic deposition of the binding agent system.
[0054] The hardeners used, which are constituents of the organic binding
agent system in the aqueous phase (A), are either a phenolic resin (4,4'-
isopropylidenediphenol, GP-Phenolic Resin BKS 7550, Ashland-Sudchemie-
Kernfest) or an isocyanate resin (Vestagon B1530, Evonik Co.) (see Table 1).
[0055] Corrosive infiltration values after 504 hours of NSS testing for the
respective organic coating on sheet steel, applied and cured in the method
presented above, may be gathered from Table 1.
[0056] It is evident that even small quantities of copper ions in the acidic
aqueous composition (B) in the method according to the present invention
bring about a significant improvement in infiltration values, as is apparent
from
a comparison of examples C1 and El, C2 and E6, and C3 and E10. The
addition of copper ions is especially advantageous for the corrosion
resistance
of the steel surfaces equipped with the cured organic coating in a context of
high Zr concentrations in the acidic aqueous composition. Increasing
concentrations of copper ions gradually result again in a deterioration in
corrosion resistance (examples 131 to B5); for the binding agent system having
the isocyanate resin as a hardener, a deterioration in the infiltration values
as
compared with a reaction rinse that contains only H2ZrF6 and no copper ions
can already be detected above 100 ppm (examples C1 and E5).
16
CA 02774106 2012-03-13
[0057] Corrosive infiltration on steel panes that were autophoretically
coated with a binding agent system, post-treated in a reaction rinse with an
acidic aqueous composition, and thermally cured.
Acidic composition (B), Neutral salt
Example Hardener in binding agent system of pH 4 spray test*
aqueous phase (A) Zr 1 [ppm] Cu [ppm] Infiltration [mm]
Cl Isocyanate resin 400 - 5.0
C2 Phenolic resin 400 - 4.5
C3 Phenolic resin 1200 - 6.0
El Isocyanate resin 400 5 3.5
E2 Isocyanate resin 400 10 3.0
E3 Isocyanate resin 400 20 3.5
E4 Isocyanate resin 400 50 4.0
E5 Isocyanate resin 400 120 5.5
E6 Phenolic resin 400 3 3.0
E7 Phenolic resin 400 5 3.0
E8 Phenolic resin 400 10 3.0
E9 Phenolic resin 400 20 4.0
E10 Phenolic resin 1200 3 4.0
Ell Phenolic resin 1200 5 4.0
E12 Phenolic resin 1200 10 4.0
E13 Phenolic resin 1200 20 4.0
per DIN 50021
' as H2ZrF6
2 as Cu(N03)2
Isocyanate resin: this organic coating was cured for 40 minutes at 185 C
after treatment with composition (B)
Phenolic resin: this organic coating was cured for 25 minutes at 150 C
after treatment with composition (B)
17
